Skrull?
That'd be weird, 'cause the Chitauri are basically the Ultimate Universe Skrulls. The Ultimate Universe is where a lot of Marvel movies seem to take their cues. Then again, we haven't see the Chitauri shape shift, so maybe Skrulls?
Phase Two should be leading up to Thanos. The Guardians of the Galaxy are basically an anti-Thanos task force, I think. Early word is that Ronan the Accuser will feature prominently and he's Kree. And the Kree/Skrull War was a HUGE event in Avengers History.
Also, if Kree, then Captain Marvel!
oh...excuse me, I have to go change my nerd pants now.